1. Field of the Invention
The present invention is directed to a system for reducing power consumption of networked wireless terminals. In particular, the invention is directed to a system which divides the wireless terminals into sets, and which instructs the sets of terminals when to switch from a low-power mode to a data-receiving mode in order to receive data. The invention has particular utility in connection with wireless ATM networks, which specify transmitting and receiving terminals in a time-division multiple access frame, and which arbitrate access thereto using a central controller, base station, or the like.
2. Description of the Related Art
ATM ("asynchronous transfer mode") is a protocol which was developed to address problems associated with transmitting multimedia data between networked devices. In particular, ATM networks are systems that negotiate and establish transmission parameters (e.g., bandwidth) prior to connecting two networked devices, "packetize" different types of data (e.g., video and audio data) into cells based on the established transmission parameters, and then multiplex these cells so that they can be transmitted over a single communication line to a receiving device. The receiving device then checks the transmitted data for errors and, if any are present, requests retransmission of the data by the transmitting device.
Traditionally, ATM networks were wire-based, meaning that devices therein were interconnected using fiber optic cables or the like. Recently, however, wireless ATM networks have been developed which replace at least some of these fiber optic cables with point-to-point wireless connections, such as radio-frequency ("RF") and infrared ("IR") links. A wireless ATM network of this type is described in U.S. patent application Ser. No. 08/770,024, entitled "Medium Access Control (MAC) Protocol For Wireless ATM", the contents of which are hereby incorporated by reference into the subject application as if set forth herein in full.
In detail, the foregoing U.S. patent application describes a communications protocol (i.e., the MAC protocol) for wireless ATM networks, which increases network quality of service, particularly in terms of allocated bandwidth, by first reserving and then scheduling resources required for data transmission. FIGS. 1 to 4 show different configurations of wireless ATM networks on which the MAC protocol may be implemented. Specifically, FIGS. 1 and 2 show centralized, or "base station", architectures, in which base stations ("BSs") control communications among various wireless terminals ("WTs"), and FIG. 3 and 4 show distributed, or "ad-hoc", architectures, in which one of the WTs is assigned the task of controlling communications. This particular WT is known as the central controller ("CC").
In both the base station and ad-hoc configurations, communications among the various WTs are effected via a periodic time-division multiple access ("TDMA") frame. In the context of the MAC protocol, this TDMA frame is known as the control-data frame ("CDF"). In general, this CDF includes both a control phase and a data phase, each of which includes plural slots (i.e., time intervals) for transmitting requests and/or data between BSs/CCs and various WTs. In the ATM context, each slot is typically equal to the amount of time required to transmit an ATM cell.
In operation, during the control phase a WT sends a request to a scheduler in a BS/CC via a control slot in the CDF. Generally speaking, this is a request for permission for the WT to transmit data cells to another WT during the data phase of a next CDF. The scheduler gathers all such requests from various WTs, and then allocates available data slots in the next CDF accordingly. That is, the scheduler allocates each data slot to a transmitting WT, such that the transmitting WT is permitted to send data in a particular data slot. Generally speaking, it is not necessary to specify particular receiving WTs, since each of these is provided with a mechanism, e.g., in their ATM layers, to determine whether a cell is addressed thereto, to accept such cells, and to disregard other cells. In any case, once slots are allocated to transmitting WTs, the allocations are broadcast to the various WTs in a "reservation message" which informs each transmitting WT that has submitted a request which data slots in the next CDF it can use to transmit data. Thereafter, in the next CDF, the transmitting WTs transmit data in their assigned slots.
One problem with the foregoing system is that all receiving WTs must remain at full power during the entire CDF, since there may be data at the end of the CDF that is designated for receipt by that WT. While this is generally not a problem in the case of wired terminals, due to the relatively unlimited supply of power from an electrical outlet, this can be problematic in WTs, which are typically battery-powered and, hence, have a limited supply of power. Recognizing this problem, power saving features have been incorporated into existing wireless networks, and into wireless ATM networks in particular. For example, in one conventional system, the BS/CC issues a reservation message specifying which WT is to transmit data to a particular slot, and also which WT is to receive data from a particular slot. With this information in hand, the receiving WTs are able to switch between a low-power (i.e., a "power-saving") mode and a higher power (i.e., "data-receiving") mode, during which data from appropriate slots can be received.
While the foregoing conventional systems can result in power conservation in the WTs, those systems also have significant drawbacks. In particular, they result in an increase in network overhead due to the additional computational effort required on the part of the BS/CC to determine the identities of both the transmitting and receiving WTs. In addition, since each receiving WT is switched between modes individually, the reservation message must include switching information for each individual receiving WT. For example, a typical reservation message may look something like: &lt;transmit_WT_id, time_of_transmission, slot#1, rcv_WT_id1_slot#1, rcv_WT_id2_slot#1, . . . , slot#2, rcv_WT_id1_slot#2, rcv_WT_id2_slot#2, . . . &gt;. Transmission and receipt of a message this complex increases network overhead and thus further decreases network efficiency.
Moreover, the additional processing capabilities required to implement the foregoing power-saving scheme, particularly at the WTs, may require additional power, which counters the amount of power actually saved. For example, the MAC layer at a transmitting WT may be unaware of the intended receiving WTs and, therefore, will need to determine this information via an additional control mechanism between itself and higher layers (e.g., the ATM layer), and by examining the ATM headers of each received packet. In addition, there may be problems with controlling the receiving WTs. For example, if a transmitting WT transmits a large number of slots, only a subset of which are destined to a specific receiving WT, PHY equalization and tracking requirements may not allow that receiving WT to switch itself on and off between specific slots.
In view of the foregoing, there exists a need for a system which reduces power consumption by WTs in a network, and which does so without significantly increasing network overhead.